The present invention relates to a bipolar transistor having a semiconductor body and an emitter region which in the body is surrounded by a base region, said two regions adjoining a major surface of the said body, two conductive contacts being provided at said major surface, namely a first contact bearing on the said emitter region and a second contact bearing on the said base region, the said transistor comprising in its emitter region an integrated resistive zone having a closed geometric configuration, the said configuration being parallel to the said major surface.
The invention relates in particular to a bipolar power transistor suitable for operation at high frequencies and having a resistor in its emitter region.
It is known that the operation of power transistors is impeded by a phenomenon known as "secondary breakdown". Briefly, this phenomenon is connected with a concentration of electric exchanges at the level of the emitter-base junction of a transistor in sharply localized and comparatively small areas of said junction; such a concentration creates local heating which produces breakdown which may lead to the destruction of the said junction.
Various solutions have been recommended in the past to avoid thermal load leading to breakdown of the junction. The most attractive among these solutions consists of interposing a resistive zone in the emitter-base current path of the transistor, either in the emitter region, or in the base region of said transistor; it is also possible to provide a resistive zone in each of the said regions.
An interesting embodiment using a resistive zone included in the emitter region of a transistor is suggested in German Patent Application No. 2,332,144.
The transistor described in said Patent Application comprises an emitter region, approximately 10 .mu.m deep, diffused in an epitaxial layer having a thickness of 25 .mu.m forming the base region, said epitaxial layer bearing on a solid substrate constituting the collector region of the said transistor. The emitter-base junction extends to the surface of the said epitaxial layer and bounds an emitter area on the central portion on which a conductive contact is provided. All around said contact a groove extends which has been cut in the emitter region down to a depth in the order of 6 to 7 .mu.m. A conductive base contact bears on the surface of the said epitaxial layer outside the emitter region. The greater part of the electric current which passes between the emitter and base contacts of the transistor hence must necessarily flow below said groove in a deep zone of the emitter region of which the height is reduced--3 to 4 .mu.m--with respect to that--10 .mu.m--of the remainder of the emitter region. The deep zone is comparatively lightly doped with respect to the surface portion, traverses the groove and is hence sufficiently resistive. Said deep zone thus constitutes a resistive zone which is integrated in the emitter region and is situated, as is desired, in the emitter-base current path of the transistor.
A transistor having a resistive emitter zone thus manufactured is satisfactory with respect to resistance to the secondary breakdown phenomenon. Unfortunately, the manufacture of a transistor of this type is impeded by the necessity of making a groove in the emitter region so as to form the resistive zone. In fact it involved a delicate operation, on the one hand because it is effected in a very sensitive region of the transistor which has a small volume, and on the other hand and in particular because of the necessary dimensional precision: the width and the depth of the groove made depend the width and the depth of the resistive zone and hence the resistance value of said resistive zone.